<select *ngIf="multiple==null || multiple==false" [(ngModel)]="myModel" class="form-control">
  <option *ngFor="let opt of ListData" value="{{opt.id}}">{{opt.name}}
  </option>
</select>

<select *ngIf="multiple!=null && multiple==true" [(ngModel)]="myModel" class="form-control" multiple>
  <option *ngFor="let opt of ListData" value="{{opt.id}}" selected="{{myModel!=null && myModel.indexOf(opt.id)>-1}}">{{opt.name}}
  </option>
</select>